Technology for integrating metals and resins is needed in many different fields of industry, such as manufacturing of parts for automobiles, consumer electrical products, industrial machinery and so forth and many adhesive agents have been developed for this purpose. Some very excellent adhesives have been proposed. For example, adhesives that exhibit their function at normal temperature or with heating are used to integrally join metals and synthetic resins and this method is currently a common joining technique.
On the other hand, more rational joining methods that do not involve the use of an adhesive have been studied heretofore. An example is a method in which a high-strength engineering plastic is integrated with a light metal such as magnesium, aluminum or an alloy of these or an iron alloy such as stainless steel without using any adhesive. For instance, the inventors proposed a method, in which a molten resin is injected onto a metal part preliminarily inserted into a metallic mold for injection molding thereby forming a resin portion and at the same time the molded article and the metal part are joined (hereinafter this will be referred to as “injection joining”).
According to the technology, a manufacturing technique was proposed, in which a polybutylene terephthalate resin (hereinafter referred to as “PBT”) or a polyphenylene sulfide resin (hereinafter referred to as “PPS”) is joined by injection joining to an aluminum alloy (see Japanese Patent Application Laid-open No. 2004-216425: Patent Document 1, for example). A joining technique was also disclosed, in which somewhat large holes (but invisible to the naked eye) is made in an anodized film on a piece of aluminum and a synthetic resin is made to penetrate into these holes and adjoined there (see WO/2004-055248 A1: Patent Document 2, for example).
The principle behind the injection joining in Patent Document 1 is as follows. An aluminum alloy is immersed in a dilute aqueous solution of a water-soluble amine compound and the aluminum alloy is finely etched with a weakly basic aqueous solution. It was found that the amine compound molecules are adsorbed to the surface of the aluminum alloy at the same time in this immersion treatment. After undergoing this immersion treatment, the aluminum alloy is inserted into a metallic mold for injection molding and a molten thermoplastic resin is injected under high pressure.
Here, the amine compound molecules adsorbed to the surface of the aluminum alloy encounter the thermoplastic resin to produce a chemical reaction such as an exothermic reaction or a macromolecular cleaving reaction. As a result of this chemical reaction, the thermoplastic resin, which was apt to be quenched, crystallized and solidified by contact with the aluminum alloy held at a low temperature of the mold, is not solidified as quickly and gets into ultrafine recesses on the aluminum alloy surface. Consequently, with a composite composed of an aluminum alloy and a thermoplastic resin, the thermoplastic resin is securely joined (hereinafter also referred to as fixed) without being separated from the aluminum alloy surface. That is, when an exothermic reaction or a macromolecular cleaving reaction occurs, a strong injection joint is produced. It has actually been confirmed that PBT or PPS, which can undergo the above-mentioned chemical reaction with an amine compound, can be joined by injection joining to an aluminum alloy. Another well known technique involves performing chemical etching preliminarily, then inserting a metal part into the metallic mold of an injection molding machine and performing injection molding with a thermoplastic resin material (see Japanese Patent Application Laid-Open No. 2001-225352: Patent Document 3, for example).
However, although the joining principle in Patent Document 1 by the inventors does exhibit an extremely good effect with aluminum alloys or the like, it has not effect in injection joining to other metals besides aluminum alloys. Accordingly, there has been a need for the development of a novel technique for joining metals and resins. The inventors discovered such a novel technique in the course of making improvements to injection joining of a hard resin to an aluminum alloy. Specifically, the conditions were discovered under which injection joining might be possible without any chemical adsorption of the amine compound to the metal part surface, in other words, without the help of a special exothermic reaction or any particular chemical reaction.
At least two conditions are necessary. The first condition is that a hard resin of high crystallinity be used, namely, that PPS, PBT or an aromatic polyamide be used and, furthermore, that these be suited to injection joining to obtain a further improved composition. Another condition is that the surface layer of the metal part have a suitably rough shape and that the surface be hard. In ordinary words, this means that the surface is strong and strength is expressed in terms of material mechanics by tensile strength, compression strength, shear strength and so forth. However, the actual thickness of the surface layer to which attention is paid in the present invention is from ten to a few dozen nanometers and the strength of such a fine portion can be rephrased as hardness. Therefore, the surface layer is preferably a ceramic layer whose hardness is higher than that of metal crystals and, more specifically, the inventors attained the conclusion that it is essential for the surface layer to be a metal oxide or metal phosphorus oxide.
For example, when a shaped material in which a copper alloy serves as the substrate is used and it is immersed in an acidic hydrogen peroxide aqueous solution, the copper is oxidized to become copper ions. As a result, if the immersion conditions are suitably set, the surface of the substrate is chemically etched to a surface roughness in which the bumps have a period of one to several microns. If the chemically etched copper alloy that has been shaped is then immersed in a strongly basic sodium chlorite aqueous solution, the copper is oxidized but the copper ions do not dissolve and the surface is covered with a thin layer of cupric oxide. Examination of this surface with an electron microscope revealed it to be covered by a ultrafine textured face in which recesses (openings) with a diameter of several dozen to several hundred nanometers are present at a period of several hundred nanometers.
These shaped copper alloys with their surfaces treated are considered theoretically as follows, assuming that they are inserted into a metallic mold for injection molding. The metallic mold for injection molding and the inserted shaped copper alloy are generally held at a temperature that is at least by 100° C. below the melting point of the resin being injected, although it varies with the injection molding conditions, so there is a high possibility that the temperature of injected resin may have dropped below its melting point at the time when it is quenched upon entering the channel inside the metallic mold for injection molding and comes into contact with copper alloy part.
Regardless of the crystalline resin, when it is rapidly cooled to below its melting point, it does not become crystallized and solidified immediately (that is, in zero time) but there is a time, albeit an extremely short time, for the resin to remain in a molten state below the melting temperature or, in other words, in a super-cooled state. If the roughness (surface roughness) of a shaped alloy is on the micron order, that is, if the recesses are large with an inside diameter of several microns, then microcrystalline resin will penetrate into these recesses within the limited time from super-cooled state to creation of the initial crystals, that is, microcrystals. To put this in another way, if the numerical density of the macromolecular microcrystal group that is produced is still low, then the resin will sufficiently penetrate into the recesses as long as the recesses are large with an inside diameter of several microns.
These microcrystals of the injected resin are surmised from molecular models to have a size from a few to more than a dozen nanometers. If there are fine openings (holes in the recesses) about 50 nm in diameter in the inner walls of the above-mentioned micron-order recesses, then there is a slight possibility of penetration, although it can hardly be said that microcrystals can readily penetrate these fine openings. Specifically, countless microcrystals are simultaneously produced, so there is an abrupt increase in the viscosity of the resin flow at places abutting on the metal face of the mold or at the distal end of the injected resin. Therefore, this resin flow is surmised to have a shape resembling the roots of a plant that stick slightly into the fine openings in the inner wall faces.
In other words, the flowing molten resin cannot penetrate into the deep portions of the fine openings but does penetrate somewhat, then crystallizes and solidifies to become a crystalline resin that has solidified in the micron-order recesses. In addition, if the metal surface layer that forms the fine openings is copper oxide, that is, a hard ceramic surface layer, then the resin will be hooked more securely within the recesses, making it less likely that the resin having solidified and crystallized will come out of the recesses. In short, the joint strength will be higher.
Improving the resin composition that is injected is an important element in the present invention. Specifically, if the resin composition is one that crystallizes slowly in injection molding (when quenched from a molten state to a temperature below the melting point), the joint strength will be higher. This is a requirement for a resin composition to be suitable for injection joining. Based on this, the inventors discovered that, if the surface of a shaped copper alloy is chemically etched as discussed above, the surface layer is made into a ceramic by a surface treatment such as oxidation and a hard crystalline resin is joined by injection joining to this, good joining ability is obtained (PCT/JP2007/070205). The inventors have also made a proposal based on their finding that, in addition to the above-mentioned PBT-based and PPS-based resins, a resin composition whose main component is an aromatic polyamide resin is also a suited to injection joining as a resin composition which is hard and highly crystalline and crystallizes extremely slowly during quenching similarly as in the technology discussed just above (PCT/JP2006/324493).
In the above description about the theory of injection joining, there is nothing that limits the kinds of metal. This indicates that injection joining can be performed using PBT, PPS or other such crystalline resin that has been improved for use in injection joining with respect to all metals and metal alloys, as long as it has the same surface shape and surface layer properties. Patent Document 3 discloses a method for manufacturing a lead wire-equipped battery cover having a shape such that several copper wires pass through the middle portion of a PPS disk, in which a chemically etched copper wire is inserted into a metallic mold for injection molding, and PPS is injected. This technology is said to be characterized by the fact that even if the internal pressure of gas generated in a battery rises a labyrinth effect will prevent the gas from leaking out through the lead wire part owing to the shape of bumps (roughness) on the surface of a copper wire by chemical etching.
At first glance the technology discussed in Patent Document 3 represents one that is similar to that according to the present invention. However, it is not the above-mentioned injection joining technology that the inventors assert in detail but is instead a technology that is an extension of existing injection molding technology and is no more than one that utilizes the difference in the linear coefficient of expansion of metals and the molding shrinkage of resins. In manufacturing a shaped article in which a rod-like metal piece passes through the inner portion of a resin part, if the resin is injected around this rod-like piece for injection molding, then the molded article is parted from the mold for injection molding and allowed to be cooled, the rod-like piece is in such a situation as to be pressed by the surrounding molded resin portion. The reason is that the linear coefficient of expansion of a metal is at most 1.7 to 2.5×10−5° C.−1 for an aluminum alloy, magnesium alloy, copper or copper alloy and, even if the calculation is made on the assumption that the metal has been removed from the metallic mold for injection molding and cooled to room temperature, the shrinkage will be in a range of the linear coefficient of expansion multiplied by a hundred and several tens of degrees and it will be no more than 0.2 to 0.30 of the total length.
Concerned with a resin, however, the molding shrinkage is about 1% for PPS and 0.5% for PPS containing glass fiber and even for a resin, in which the filler content has been increased, the resin portion will always undergo more heat shrinkage than the metal part after injection molding. Therefore, if a shaped article in which the metal part is disposed in the center and this metal part goes through the resin portion is produced by injection molding with an insert, an integrated product can be manufactured in which the metal part is not likely to come loose due to the pressing effect produced by heat shrinkage after the molding of the resin portion.
This method for manufacturing an integrated metal and resin product that makes use of heat shrinkage is known conventionally and is used to fabricate knobs on fuel oil stoves, for example. This method involves inserting a thick iron needle with a diameter of about 2 mm into a metallic mold for injection molding and injecting a heat resistant resin or the like into the mold. In this method, jagged bumps (by knurling) are formed around the outer peripheral face of the needle and the resin is injected and molded so that there is no relative movement. Patent Document 3 discloses that the surface configuration is smoothed by changing the texturing process from a physical process to a chemical process with knurling or the like, bumps are made finer and grip effect is improved by using a resin that is hard and crystalline.
The composite according to the present invention does not at all require that the resin press the metal by heat shrinkage or the like, and even with a shaped article in which two flat plates are joined together at their flat planes, a tremendous force is needed to break the joint. In order that the joined state of the metal and thermoplastic resin is to be maintained stably over an extended period, it is actually necessary for the linear coefficients of expansion of the two materials to be close in value. The linear coefficient of expansion of a thermoplastic resin composition can be lowered considerably by adding a large amount of glass fiber, carbon fiber or other such reinforcing fiber (that is, a filler) but the limit to this is generally 2 to 3×10−5° C.−1. Kinds of metals that have such numerical value at a normal temperature or so are aluminum, magnesium, copper and silver.
The present invention relates to technology that makes possible the injection joining of a hard resin to stainless steel. The linear coefficient of expansion of stainless steel is about 1×10−5° C.−1, which corresponds to about the middle value of the above-mentioned group of metals. In this sense, research and development related to injection joining conducted by the inventors lags behind in priority, while it is thought very likely that it can be used if the temperature range in use is narrow and the inventors have also conducted research and development into stainless steel.
Stainless steel has a specific gravity of about 8. It has high mechanical strength and is used as a metal with high corrosion resistance. Therefore, stainless steel parts are frequently used in various heavy-duty electronic and electrical equipments, medical instruments, automotive mounted equipments, automobile parts, marine machineries and other such parts used in movable equipments and particularly in the casings and housings of equipments that may be exposed to drops of salt water or sea water. If a hard resin can be injected onto stainless steel, the production of these casings or housings for equipments is considered to be extremely easy.
The required conditions for the injection joining of a metal and a resin will once again be summed and explained below based on the hypothesis of the inventors. Specifically, to obtain good injection joining strength, at least the shaped metal should satisfy the following conditions.
(1) The surface has large bumps (surface roughness) obtained by chemical etching and the period thereof is usually on the micron order, which in the present invention refers to the range of 0.5 to 10 μm.
(2) The surface is sufficiently hard (a metal oxide or metal phosphorus oxide) and, to prevent slippage, has a coarse surface that consists of ultrafine bumps on the nanometer order (a coarse surface in subjective view with an electron microscope).
(3) The resin must be a crystalline resin of high hardness, while it is particularly favorable to use these improved compositions in which the crystallization rate during quenching is further slowed.
The findings of the inventors have shown that this hypothesis is correct for magnesium alloys, copper alloys, and titanium alloys. The “coarse surface” in (2) above is a figure of speech expressing what is observed with an electron microscope and high injection joining strength can be obtained when the surface is a ultrafine textured surface in which the spacing period is at least 10 nm and the height or depth was at least 10 nm.